I think that a 1991 would have a wood frame, but not for sure. You can call 1-800-835-0676 and ask for RV, if you give him the last 5 numbers of the vin, he should be able to tell you anything you want to know about the fifth-wheel.
Good luck on your new to you fifth-wheel.
2006 Ford F-350 Crew Cab Lariat 4X2 PSD 6.0 TorqShift Automatic Transmission. Upfitter Switchers Chrome Door Handles, Oxford White with lower part being Arizona Beige Metallic. Creative Coating Sprayed in bed liner (local Company in Little Rock AR) 2